The present invention relates to radar transmitters and, more particularly, to control circuits for regulating feedback from the output of the traveling wave tube so as to augment the radio frequency drive from the exciter source and induce the traveling wave tube to drive itself into saturation.
In a radar transmitter it is generally desirable to maintain the power output level of the radar interrogating signal constant, and as high as possible as the frequency of the interrogating signal is modulated. Thus, the amplitude of the return signal is not a time varying function related to the interrogating signal, but rather a function of the composition and movement of the area being interrogated. Attainment of constant high output power and low amplitude modulation noise levels, depend upon the traveling wave tube (TWT) being operated at saturation, i.e., at the highest output level that the TWT can deliver. The radio frequency drive level required to achieve saturation, in turn, depends upon many parameters that may vary on a dynamic basis. Those parameters include the instantaneous RF frequency, TWT power supply and control voltages, temperature, RF drive path losses, TWT variations from tube to tube, aging of the TWT, and localized magnetic fields.
In addition to dynamically varying parameters, the RF drive level required to achieve saturation also depends upon the output mode in which the TWT is operating. Contemporary dual mode radar systems operate at high and low output power levels in order to permit interrogation of local areas without generating a high level radar interrogation signal that can be readily intercepted at a distance. Because the TWT gain drops much faster than the TWT output power, the required input power to operate in low output power mode may be substantially greater than that required to operate in a high power mode. Therefore, contemporary TWT drive circuits for dual mode radar transmitters must incorporate an RF exciter source having sufficient output power to drive the TWT into saturation in the low output power mode. In order to achieve maximum useful life from the TWT, a control circuit should preferably compensate for variations in the above parameters so as to maintain the TWT in saturation.
Two types of control circuits are currently employed to maintain the TWT in saturation. The first type measures the TWT output power and utilizes feedback control to adjust the attenuation of the signal from the RF source which serves as the sole source of TWT input power. The second type calibrates and stores the proper RF drive level to saturate the TWT in each operating mode at system turn on, and then commands the appropriate level of attenuation of the signal from the RF source, which again is the sole TWT input.
In order to determine the appropriate attenuation level, contemporary feedback control circuits sense the output power of the TWT and adjust an attenuator in the TWT's RF drive line when the output power varies, e.g., in response to a change in the RF frequency. Because such contemporary feedback control circuits sense the average output power of the TWT and dither the feedback level to arrive at the optimum output power, those circuits are less sensitive to the TWT operations and result in the introduction of noise lines into the TWT input signal. The noise component of the TWT input signal causes irregulaties in the output power level which degrade the circuit's ability to maintain a constant output power level from the TWT. Moreover, control circuits that operate by variable attenuation of the RF source signal level requires a sufficiently high power level RF signal source to drive the TWT into saturation in any operating mode.
In general, the reaction time of TWT feedback control circuits is limited by the operating frequency of the TWT and time delays inherent in the TWT. More particularly, the feedback signal must arrive at the input of the TWT within a 90.degree. phase angle variation of the exciter source signal. Otherwise, the feedback signal is too far out of phase with the exciter signal to exert required control and stabilizing influence. Because contemporary TWT's have inherent transmissions delays of approximately 20 nano seconds (20 ns), the feedback signal is a minimum of 20 ns behind the exciter signal. Assuming the limiting condition that the 20 nano second period represents 90.degree. of the maximum RF input signal, the highest RF signal frequency at which a feedback control circuit could operate is approximately 12.5 MHz. Small delays resulting from implementation of calibration routines and power setting will further increase the minimum feedback time, and therefore reduce the highest RF frequency at which the system can operate. Moreover, as with contemporary feedback control circuits, such calibration circuits require control devices for rapidly adjusting the RF drive signal. Those devices can contribute substantially to the cost of the TWT circuit.